1. Field of the Invention
The present invention relates to an alkaline storage battery such as a nickel-metal hydride battery, a nickel-cadmium battery, and the like, and to a nickel electrode for alkaline storage battery used for a positive electrode of the lkaline storage battery as described above, and is particularly characterized in that charge/discharge cycle performance of the alkaline storage battery under high temperature conditions is improved upon bettering the nickel electrode for alkaline storage battery formed by applying a paste containing active material particles composed of nickel hydroxide to a conductive substrate and then drying the paste on the conductive substrate.
2. Description of the Related Art
A conventional alkaline storage battery such as a nickel-metal hydride battery and nickel-cadmium battery, has used as its positive electrode a nickel electrode for alkaline storage battery using nickel hydroxide as an active material.
In the above-mentioned nickel electrode for alkaline storage battery, conductivity of the nickel hydroxide used as the active material is low. Accordingly, a sintered nickel electrode formed by impregnating a substrate prepared by filling nickel powder into a conductive substrate such as porous steel sheet and the like and then sintering the conductive substrate having the nickel powder filled therein, with nickel hydroxide as the active material has been conventionally used.
However, in such sintered nickel electrode, close adherence between particles of the nickel powder is weak. Accordingly, the nickel powder drops out easily when the substrate having high degree of porosity is used. Therefore, the maximum porosity of such substrate is 80% in actual conditions, and hence, the nickel hydroxide as the active material is not sufficiently filled, thus an alkaline storage battery having a large capacity was hardly attained.
Further, in the above-mentioned sintered nickel electrode, substrate including the porous steel sheet is used, thus, filling density of the active material is generally small. In addition, a pore diameter of the nickel powder formed by sintering is generally small, for example, not more than 10 xcexcm. Thus, in filling the active material into the substrate, solution impregnating method in which laborious work is repeatedly performed for cycles must be taken, thereby degrading productivity.
Therefore, a paste type nickel electrode for alkaline storage battery formed by applying a paste which is obtained by mixing the active material particles composed of nickel hydroxide with an aqueous solution as a binding agent such as methyl cellulose to a conductive substrate having the high degree of porosity such as foamed nickel and drying said paste has been used.
In such paste type nickel electrode for alkaline storage battery, the conductive substrate having the porosity of not less than 95% can be used. Accordingly, a large number of active materials can be filled into the conductive substrate, thus the alkaline storage battery having the large capacity is attained, and the active materials can be easily filled into the conductive substrate, thereby improving the productivity.
However, in such paste type nickel electrode for alkaline storage battery, when the conductive substrate having the high degree of porosity is used to fill the large number of active materials therein, current collectivity of the conductive substrate is degraded, thereby reducing the utilization efficiency of the active materials.
Therefore, in recent years, in such paste type nickel electrode for alkaline storage battery, a method in which metal cobalt or a cobalt compound composed of a cobalt oxide or a hydroxide as a conductive agent are added to the above-mentioned active material particles composed of nickel hydroxide, then the above-mentioned metal cobalt or the cobalt compound are oxidized to xcex2-CoOOH which is cobalt oxyhydroxide by charge, to increase the conductivity of the electrode, thus to improve the utilization efficiency of the active materials has been used.
However, even in a case in which the metal cobalt or the cobalt compound as the conductive agent are added to the active material particles composed of nickel hydroxide, there still have remained problems that when the paste type nickel electrode for alkaline storage battery is used as the positive electrode of the alkaline storage battery and is charged under high temperature conditions, an oxygen overvoltage in the positive electrode is decreased, thus in addition to a charge reactivity in which nickel hydroxide is oxidized to nickel oxyhydroxide, a side reaction in which an oxygen evolution reactivity occurs and hence, charge characteristics is decreased have occurred.
In this connection, Japanese Laid-Open No. Shou 57-187869 proposes to add at least one of metal titanium, titanium oxide, and titanium hydroxide to the active material composed of nickel hydroxide as well as to use an alkaline electrolyte solution containing lithium ion in order to improve charge/discharge efficiency of the alkaline storage battery upon improving the utilization efficiency of the nickel electrode.
However, in the alkaline storage battery in which at least one of metal titanium, titanium oxide, and titanium hydroxide is added to the active material composed of nickel hydroxide and alkaline electrolyte solution containing lithium ion is used, there have remained problems that the current collectivity of the electrode is degraded, and sufficient discharge capacity is not attained.
An object of the present invention is to increase current collectivity and to improve utilization efficiency of active material of a nickel electrode for alkaline storage battery formed by applying paste containing active material particles composed of nickel hydroxide to a conductive substrate and then drying the paste on the conductive substrate.
Another object of the present invention is, in a case in which an alkaline storage battery using as its positive electrode the above-mentioned nickel electrode for alkaline storage battery is charged and discharged under high temperature conditions, to prevent discharge capacity of the alkaline storage battery from gradually decreasing and to improve charge/discharge cycle performance under high temperature conditions.
The nickel electrode for alkaline storage battery of the present invention is formed by applying the paste containing the active material particles composed of nickel hydroxide to the conductive substrate and then drying the paste on the conductive substrate, wherein a conductive layer consisting of sodium-containing cobalt oxide is formed on a surface of the above-mentioned active material particles, and titanium powder and/or titanium compound powder is added to the active material particles.
Further, when the conductive layer consisting of sodium-containing cobalt oxide is formed on the surface of the active material particles composed of nickel hydroxide as the nickel electrode for alkaline storage battery of the present invention, the current collectivity inside the electrode becomes higher, thus the utilization efficiency of the active material is improved, because electrical conductivity of the sodium-containing cobalt oxide is higher than that of metal cobalt or cobalt compound.
When charge/discharge is performed to the alkaline storage battery using as its positive electrode the nickel electrode for alkaline storage battery under high temperature conditions, the sodium-containing cobalt oxide is prevented from being reduced to the cobalt hydroxide during discharge and dissolving into an alkaline electrolyte solution of the alkaline storage battery.
In addition, when the titanium powder and/or titanium compound powder is added to the active material particles which are composed of nickel hydroxide, and on which the conductive layer consisting of sodium-containing cobalt oxide is formed, the speed for which the cobalt hydroxide dissolves into the alkaline electrolyte solution and deposits is delayed for the effect of the titanium and/or titanium compound even in the case in which a part of sodium-containing cobalt oxide is reduced the cobalt hydroxide. As a result, the cobalt hydroxide is prevented from segregating on the surface of the active material particles, and hence, a part of the cobalt hydroxide is restrained from diffusing in the pore of the active material particles, thus, the charge/discharge cycle performance under high temperature conditions is improved.
In the nickel electrode for alkaline storage battery of the present invention, in forming the conductive layer consisting of sodium-containing cobalt oxide on the surface of the active material particles composed of nickel hydroxide, metal cobalt powder, cobalt hydroxide powder, cobalt monoxide powder, and cobalt oxyhydroxide powder are mixed with the active material particles to prepare a mixture. Alternatively, the layer consisting of metal cobalt, cobalt hydroxide, cobalt monoxide or cobalt oxyhydroxide is formed on the surface of the active material particles. Afterward, sodium hydroxide aqueous solution is added to the aforementioned resultant mixture or layer, and then is subject to heat-treating at the temperature of 50 to 200xc2x0 C. under the presence of oxygen, to form the above-mentioned conductive layer.
In heat-treating, the temperature is set in the range of 50 to 200xc2x0 C. because in the case in which the temperature is not more than 50xc2x0 C., CoHO2 which is low in the electric conductivity deposits, while in the case in which the temperature is not less than 200xc2x0 C., 3-cobalt tetraoxide Co3O4 which is low in the electric conductivity deposits, accordingly in both cases, the conductive layer having high conductivity is not attained. When the cobalt oxyhydroxide particles are added to the surface of the active material particles, or the layer consisting of the cobalt oxyhydroxide is formed on the surface of the active material particles, CoHO2 does not deposit even in the case in which the heat-treating temperature is not more than 50xc2x0 C. However, sodium is hardly contained, accordingly the conductive layer having the high conductivity is not attained. Time for the above-mentioned heat-treating is not especially limited but is altered appropriately depending on concentration of the sodium hydroxide to be used or the heat-treating temperature. The time is approximately set in the range of 0.5 to 10 hours.
Further, in the case in which the conductive layer consisting of sodium-containing cobalt oxide is formed on the surface of the active material particles composed of nickel hydroxide as mentioned above, a chemical structure of the sodium-containing cobalt oxide is uncertain. However, electric conductivity thereof is extremely high, therefore, the sodium-containing cobalt oxide is expected to be not a mere mixture of cobalt oxide and sodium but an intercalation complex having a structure of sodium being interposed into cobalt oxide crystals.
The above-mentioned layer consisting of metal cobalt, cobalt hydroxide, or cobalt monoxide is formed on the surface of the active material particles composed of the nickel hydroxide by mechanical charging method in which metal cobalt powder, cobalt hydroxide powder, or cobalt monoxide powder is added to the nickel hydroxide powder, and then said nickel hydroxide powder is dry mixed by a compressible crusher under inert-gas atmosphere.
The above-mentioned layer consisting of cobalt hydroxide is formed on the surface of the active material particles composed of nickel hydroxide by the steps of adding nickel hydroxide powder to a cobalt salt aqueous solution such as cobalt nitrate, dropping an alkaline aqueous solution such as a sodium hydroxide aqueous solution into an obtained mixture while agitating the obtained mixture to adjust the pH of the solution to around 11, reacting a resultant solution for an appointed time while agitating the resultant solution, and depositing cobalt hydroxide on the surface of the nickel hydroxide particles.
The above-mentioned layer consisting of the cobalt oxyhydroxide is formed on the surface of the active material particles composed of nickel hydroxide, for example, by the steps of forming the layer consisting of cobalt hydroxide on the surface of the active material particles composed of nickel hydroxide, and reacting the layer thus formed with hydrogen peroxide water which is heated to about 40xc2x0 C., and oxidizing the cobalt hydroxide
In forming the conductive layer consisting of the sodium-containing cobalt oxide on the surface of the active material particles composed of nickel hydroxide as mentioned above, when the weight ratio of the conductive layer based on the active material particles is too small, the conductivity of the nickel electrode for alkaline storage battery is not fully improved. On the other hand, when the weight ratio of the conductive layer based on the active material particles is too large, the ratio of the nickel hydroxide in the nickel electrode for alkaline storage battery is decreased, thereby decreasing the discharge capacity. Therefore, the weight ratio of cobalt element in the conductive layer based on the active material particles composed of nickel hydroxide is preferably set in the range of 1 to 10 wt %.
In the above-mentioned conductive layer consisting of the sodium-containing cobalt oxide, when the weight ratio of the sodium element in the sodium-containing cobalt oxide is too small or too large, the sodium-containing cobalt oxide is easily reduced to cobalt hydroxide during discharge under high temperature conditions as the result of both cases. Therefore, the weight ratio of the sodium element in the sodium-containing cobalt oxide is preferably set in the range of 0.1 to 10 wt %.
In adding the titanium powder and/or the titanium compound powder to the surface of the active material particles on which the above-mentioned conductive layer is formed, when an additive weight ratio is too small, the charge/discharge cycle performance under high temperature conditions is not fully prevented from decreasing. On the other hand, when the additive weight ratio is too large, the ratio of nickel hydroxide in the nickel electrode for alkaline storage battery is decreased, thereby decreasing the discharge capacity. Therefore, the weight ratio of a titanium element in the titanium powder and/or the titanium compound powder to be added based on a total weight of the active material particles composed of the nickel hydroxide and the above-mentioned conductive layer formed thereon is preferably set in the range of 0.2 to 4.0 wt %.
Examples of the above-mentioned titanium compound include TiO2, TiO, Ti2O3, Ti(OH)4, Ti(OH)2, Ti(OH)3, TiO2.xH2O.
When a particle diameter of the above-mentioned titanium powder and/or the titanium compound powder is too large, an area in which the titanium powder and/or the titanium compound powder contacts with the surface of the active material particles on which the conductive layer is formed is decreased, thus sufficient effect is not attained. Therefore, the titanium powder and/or the titanium compound powder having an average particle diameter of not more than 100 xcexcm is preferably used.
In the nickel electrode for alkaline storage battery of the present invention, at least one element selected from a group consisting of zinc, cobalt, calcium, magnesium, aluminum, manganese, yttrium, and ytterbium is preferably incorporated into the above-mentioned active material particles composed of the nickel hydroxide, and the ratio of these elements based on the total weight of the nickel in the above-mentioned nickel hydroxide and these elements is preferably set to not more than 10 atomic % to prevent the potassium ion and the like in the alkaline electrolyte solution from being intercalated into the crystal of the nickel hydroxide as the active material for the effect of these elements thus incorporated, thus to prevent the decrease of the charge/discharge capacity by dry out of the alkaline electrolyte solution. Especially, when at least one of zinc and cobalt is incorporated, the decrease of the charge/discharge capacity by dry out of the alkaline electrolyte solution is further prevented because of a greater effect of these two elements.
In addition, in the nickel electrode for alkaline storage battery of the present invention, it is preferable that at least one element powder and/or its compound powder selected from the group consisting of yttrium, ytterbium, calcium, aluminum, erbium, gadolinium, thulium, lutetium, zinc, niobium, tungsten, and tantalum in addition to titanium powder and/or the titanium compound powder is added to the surface of the active material particles on which the conductive layer consisting of sodium-containing cobalt oxide is formed. When the selected element powder and/or its compound powder is added, charge/discharge cycle performance under high temperature conditions is further improved. Especially, when at least one element powder and/or its compound powder selected from the group consisting of yttrium, niobium, tungsten, and tantalum is added, charge/discharge cycle performance under high temperature conditions is remarkably improved because of the greater effect. In particular, when Y2O3 which is an yttrium compound is added, charge/discharge cycle performance under high temperature conditions is more remarkably improved.
In the nickel electrode for alkaline storage battery of the present invention, examples of the above-mentioned conductive substrate on which the paste containing active material particles is applied include foamed nickel, felt metal fiber, and punching metal.
Further, in the alkaline storage battery using as its positive electrode the above-mentioned nickel electrode for alkaline storage battery, an alkaline electrolyte solution containing potassium, lithium, and sodium is preferably used in order to improve charge characteristic under high temperature conditions, thus to restrain oxygen evolution at excessive charge. Especially, an alkaline electrolyte solution containing 4 to 10 mol/l of potassium hydroxide, 0.1 to 2 mol/l of lithium hydroxide, and 0.2 to 4.0 mol/l of sodium hydroxide is more preferably used.
Examples of the alkaline storage battery using as its positive electrode the above-mentioned nickel electrode for alkaline storage battery include a nickel-metal hydride battery using as its negative electrode a hydrogen absorbing alloy electrode, a nickel-cadmium battery using as its negative electrode a cadmium electrode, and a nickel-zinc battery using as its negative electrode a zinc electrode.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiment of the invention.